The invention is directed to a control rod and associated drive which can be employed in fuel handling in a gas-cooled nuclear reactor.
Nuclear reactors used in electrical generating stations, as process heat sources, etc., require periodic refueling. Refueling must in many cases be accomplished while the reactor is shut down which means that the reactor is not producing useful power during the time required to add to, remove from, or shuffle nuclear fuel in the reactor core. Due to the large capital investment in this equipment, it is important that the reactor be in operation as much of the time as possible to make its operation commercially feasible. Therefore, shutdowns must be minimized in frequency and duration. Refueling shutdowns may be several weeks to several months in duration and may occur as frequently as once every twelve to fifteen months. Furthermore, some maintenance shutdowns may require removal of some or all of the fuel from the core, e.g., to examine pressure vessel welds.
Because fuel addition, removal, and shuffling activities are usually on the critical path for the shutdown, any improvement in the speed with which these tasks may be accomplished results directly in a shortening of the shutdown and hence improves the ability of the reactor to earn revenue.
The refueling of modern gas-cooled reactors involves the removal of a large weight and volume of spent fuel and moderator and its replacement by new fuel. If the reactor must be refueled while shut down, the time available for refueling is necessarily limited by the aforementioned considerations. Refueling while operating, on the other hand, while making available the whole of the plant operating time for refueling, raises serious safety questions and requires a generally higher level of complexity in the fuel-handling equipment. Current gas-cooled reactors designed for shutdown refueling circumvent the problem by refueling a relatively small portion of the core at each annual shutdown period. However, the procedure requires that radial zones of the core be differentially orificed to match the particular fuel burnup situation of the zones. Orificng is an undesirable additional complication and introduces additional core pressure drop which results in the expenditure of increased energy to circulate the primary coolant gas.
Ideally the new fuel should be added at a prescribed axial location across the whole area of the core and the remaining fuel reshuffled axially to make room for the new fuel. However, such a refueling arrangement requires large fuel-handling capability, beyond the capability of current gas-cooled reactors whose fuel is generally added and removed a block at a time through a control rod drive penetration. The system herein removes these limitations on handling capacity by making use of the control rods and their drives in the handling of the fuel. This technique permits the handling of a complete stack of large fuel blocks which greatly reduces the fuel-handling time. The end result is that the entire core of a reactor may be changed or reshuffled in the time required for a normal annular shutdown instead of just a fraction of the core. This, in turn, makes the elimination of differential orificing of radial segments of the core feasible and allows fueling of the reactor core with fuel blocks constant in their composition rather than varying depending upon their position in the core.